(1) Field of the Invention
The present invention relates to a control device and a control method of an optical amplifier, and more particularly to a control device and a control method of an optical amplifier suitably applicable to a constant output control and a constant-gain control of an optical amplifier.
(2) Description of Related Art
Today, a wavelength division-multiplexing (WDM) device, which increases a transmission capacity by inputting into one transmission line (optical fiber) a plurality of optical signals with different wavelengths, has been widespread. Moreover, in an optical ring network, as an optical transmission device (optical node), which enables the increase and decrease of an arbitrary wavelength regarding a multiplexed WDM light in the middle of the transmission line, an OADM (Optical Add-Drop Multiplex) device has also been put to practical use.
In the WDM optical-communications system described here, there is a need to control the output signal light power per one wave to be constant by controlling the amplification gain to be constant in the optical amplifier even when the wavelength number of the WDM light changes. For this reason, an optical amplifier is needed that responds at high-speed to the increase and decrease of the wavelength. In controlling optical amplifiers today, this high-speed response control is generally executed by an automatic gain control (AGC).
Moreover, under the conditions that optical amplifiers are coupled in multi-stages like a long-distance WDM optical communications system, if there occurs a change in the transmission line loss due to the temperature, or aging and the like of the transmission fibers, the transmission fibers would incur a deterioration of the transmission quality if only AGC would be used. Then, in order to absorb input fluctuations (input dynamic range) per one wave and obtain a constant signal output level (power), there is a need to execute ALC (Automatic Level Control) in addition to AGC, combined with it.
In FIG. 9, a configuration of the conventional optical amplifier, which uses such AGC combined with ALC, is shown. The optical amplifier shown in FIG. 9 is constituted as an in-line amplifier of a two-stage amplification configuration, and is comprised of: as an optical circuit section, erbium-doped optical fibers (EDF) 100a and 100b which are optical amplification media; a variable optical attenuator (VOA) 101 for adjusting the optical level (the amount of loss) provided between such EDF 100a and 100b: beam splitters (BS) 102a and 102b, photo-diodes (PD) 103 and 104, which constitute a monitoring circuit of the input/output light level of EDF 100a in the pre-stage; an excitation light source (LD) 105 and a WDM coupler (wavelength coupler) 106 which constitute an excitation light circuit for EDF 100a; BS 107a as well as 107b and PD 108 as well as 109 which constitute a monitoring circuit of the input/output light level of EDF 100b arranged in the post-stage; and an excitation light source (LD) 110 and an WDM coupler 111 which constitute an excitation light circuit for EDF 100b. 
Moreover, as a control circuit section (an electric circuit section), there are also provided an AGC circuit 121 for EDF 100a of the pre-stage, an AGC circuit 122 for EDF 100b in the post-stage, and an ALC circuit 123 which controls the amount of attenuation of VOA 101 as well as ASE correction circuits (adder) 124 and 125.
In the optical amplifier (EDFA) using such EDF, a total sum of the gain by each of EDF 100a and 100b and the loss in VOA 101 makes the gain (a total EDF gain) as the whole optical amplifier, and the total EDF gain can be changed by changing the gain of EDF 100a and 100b, or the amount of loss in VOA 101.
Specifically, in the above-described optical amplifier, a WDM light inputted from a signal input terminal is amplified in EDF 100a, and after the output level thereof (namely, an input level to EDF 100b in the post-stage) being adjusted in VOA 101, is then amplified again in EDF 100b and outputted. Then, at this time, a part of the input/output light in each of EDF 100a and 100b is branched out in BS 102a as well as 102b, and BS 107a as well as 107b, respectively, and the input/output light power of each of EDF 100a and 100b is monitored in PD 103 as well as 104, and in PD 108 as well as 109, respectively. Namely, each of PD 103, 104, 108 and 109 inputs an electric signal (a voltage value) corresponding to the input light quantity into corresponding AGC circuits 121 and 122, respectively.
In the AGC section (AGC circuits 121 and 122), an AGC is carried out including the power fluctuation of an input signal light. Namely, the output power (an excitation light power) of LDs 105 and 110 is controlled based on the input voltage value from each of PDs 103, 104, 108 and 109, such that the total EDF gain is kept at a predetermined constant value (such that the ratio of the input light level of EDF 100a in the pre-stage and the output light level of EDF 100b in the post-stage may be constant).
On the other hand, in the ALC circuit 123, in order to absorb the input fluctuation per one wave (input dynamic range) and obtain a constant output signal light level, the total EDF gain is changed in the direction of suppressing the power fluctuation of the input signal light by adjusting the amount of loss in VOA 101, based on the monitoring value of PD 109 (that is, the output light level of EDF 100b in the post-stage).
Accordingly, when the input light power fluctuates at a speed sufficiently slower than the speed of the response (time constant) of the ALC circuit 123, it is possible to completely suppress a fluctuation of the input light power within the input dynamic range and control the total EDF gain to a predetermined value.
Incidentally, as described in the following Patent Document 1, a time constant of the AGC is set sufficiently short with respect to the response time (length of time from a time of the excitation light power having changed until the gains of EDF 100a and 100b being adjusted to the desired values corresponding to the changes thereof; usually several milli seconds) of EDFs 100a and 100b, so as to sufficiently cope with the wavelength number fluctuation (the increase and decrease of the wavelength) of the input signal light, as described above. On the other hand, the time constant of the ALC is set, for example, so as to be longer (such as 10 or more times and the like) than the time required for a supervisory control signal to be transmitted to each optical node through OSC (Optical Supervisory Channel).
Incidentally, in the optical amplifier using EDF, ASE (Amplified Spontaneous Emission), which becomes a noise component along with the amplification of the input light, is generated as described in the paragraphs 0053 and 0054 of the Patent Document 1. For this reason, on the assumption that the input light power (the total power) for the optical amplifier is designated by “Pin”, the input light power per one wave to the optical amplifier by “Pin_ch”, the output light power (the total power) by “Pout”, the signal gain by “G”, the ASE output generated in its own stage by “Pase”, and the wavelength number by “m”, the gain set by the AGC is expressed by the following equation (1), and an error due to the ASE will be generated in association with the signal gain G which is a desired gain. And, this error depends on the multiplexing number of the wavelength of the WDM light.
                                                                        AGC                ⁢                                                                  ⁢                gain                            =                                                Pout                  /                  Pin                                =                                                      (                                                                  G                        ×                        Pin                                            +                      Pase                                        )                                    /                  Pin                                                                                                        =                              G                +                                  Pase                  /                                      (                                          m                      ×                      Pin_ch                                        )                                                                                                          (        1        )            
Therefore, it is apparent that the AGC gain has only to be increased in association with the signal gain G only by the amount of Pase/(m×Pin_ch). Then, in the conventional optical amplifier shown in FIG. 9, the ASE correction value (=Pase/G) is reflected on the input information (the monitoring value of the input light power of EDF 100a and 100b) to the AGC section (AGC circuits 121 and 122) by means of the adders 124 and 125. In addition, in some cases the ASE correction value is reflected on the monitoring value of the output light power of EDFs 100a and 100b (refer to FIG. 11 of the Patent Document 1). Moreover, as described in the paragraph 0058 of the Patent Document 1, as for also the ALC the ASE correction value is also reflected. In addition, the details of such ASE correction are described, for example, in the paragraphs 0055 and 0056 of the Patent Document 1.
[Patent Document 1]
Japanese Patent Laid-Open (Kokai) 2003-174421.
Incidentally, since the above-described “ASE correction value” is significantly dependent on the input light power per one wave, the “ASE correction value” always needs to be changed at the time of ALC in which the input light level per one wave is likely to fluctuate. The control value thereof is set based on the input light power per one wave level (Pin_ch), and is, therefore, the value which originally is not subjected to any change by the fluctuation of the wavelength number.
However, in the event that a fluctuation of the wavelength number has occurred during the ALC, it is assumed, until the correct wavelength number information can be obtained by the OSC or the like, that there is no changes in the wavelength number, with the result that changes of the input light power due to the fluctuation of the wavelength number are interpreted as changes in the input light level per one wave. As a consequence, an error would occur in the ASE correction value, and in configurations like the optical amplifier described above based on FIG. 9 or the technologies of the Patent Document 1, in which the ASE correction value is reflected on the desired control value of the total EDF gain or on the desired control value of VOA 101, an error would occur also in this desired control value.
Namely, in the control information (the gain of the whole optical amplifier, each EDF gain, ASE correction values, and the like) by which a desired value is set from the input light level per one wave, a control error would occur. As a result, the control value (the amount of attenuation) of VOA 101, which should originally be constant even at the time of the increase and decrease of the wavelength number, will change, and the transmission quality of the main signal light (WDM light) will degrade due to the influence of the gain deviation, and the like.
Moreover, in the device in which optical amplifiers are coupled in multi-stages, the ASE correction value generated in its own stage is notified by the OSC or the like to an optical amplifier arranged in the post-stage, and is used as the control information in this optical amplifier, however, when a fluctuation of the wavelength number occurs in the state of ALC, an accurate ASE correction value which should be generated in its own stage can not be calculated, therefore, a control using the ASE correction value at the time of the increase and decrease of the wavelength number cannot be carried out in the subsequent optical amplifier.